The thermal history of the Browse Basin and its implications for petroleum exploration
Beardsmore, Graeme, Doctoral thesis from Monash University, Australia, 1996.
Basin scale heat flow contour maps are required for petroleum exploration to accurately model organic maturation within source beds. This investigation of the thermal structure of the Browse Basin (North West Shelf, offshore Australia) assessed the applicability and limitations of present techniques of determining heat flow from exploration well data, and developed new techniques to overcome current problems.
The greatest source of error in heat flow estimates is inadequately constrained thermal conductivity. Accuracy can be substantially improved with an understanding of the relationship between lithology, porosity and thermal conductivity within a localized region. A technique was developed for calculating heat flow within petroleum exploration wells. Mud-logs, local porosity–depth data and appropriate phase mixing laws were utilized to derive thermal conductivity profiles within the wells. Subsequent heat flow estimates are thus based on local data and not global averages.
Critical to the calculation of off-shore surface heat flow is an estimate of the temperature at the sediment–water interface. Global data was collated and statistically treated to produce a model of bottom-water temperature as a function of water depth and latitude. Temperatures can be estimated to within 2°C, comparable with the accuracy of down-hole temperature estimates. This development represents an improvement in the accuracy of subsequent surface heat flow estimates.
Heat generated within sediments may significantly affect surface heat flow. The relationship between several electric well logs and heat production was investigated in detail and a model developed for estimating heat production using gamma ray and density logs. Sedimentary heat production was found to be negligible within the depth extent of individual wells in the Browse Basin.
The Horner method is sufficiently accurate for correcting bottom-hole temperature data. Corrected down-hole temperatures were combined with bottom-water temperatures and thermal conductivity models to produce a best estimate of heat flow for each of 32 petroleum exploration wells in the Browse Basin. Several wells (in particular Barcoo 1, Brewster 1A, Caswell 2 and North Scott Reef 1) have a surface heat flow significantly lower than the heat flow below the top of a thick sequence of Cretaceous shale. This anomaly can not be effectively explained by varying thermal conductivity values, and was therefore seen as evidence of heat removal by fluid flow along a limestone aquifer.
Conditions within the Browse Basin are poorly suited for most organic maturation investigations, but previously measured and new maturation data indicate that temperatures over the entire basin are currently at an historical maximum. Burial history modeling reveals that fluid flow within the limestone aquifer must therefore have had a thermal effect on the basin since shortly after deposition of the aquifer around 80 million years ago.
A heat flow history taking into account higher heat flow in the central and south regions of the Browse Basin prior to the Cretaceous indicates a maturation history of Triassic source beds significantly different from that suggested by a linear heat flow history. The former suggests the beds entered the oil window approximately 100 Ma before the latter in Barcoo 1, and approximately 40 Ma before the latter in Caswell 2. The thermal history of North Scott Reef 1 is dominated by rapid burial within the last 50 Ma, and modeled maturation histories show little variation between the two models.